This disclosure is related to the field of measurement while drilling (“MWD”) and logging while drilling (“LWD”). More particularly, the disclosure relates to improved electromagnetic telemetry techniques for communicating data between MWD and/or LWD instruments and the Earth's surface.
U.S. Patent Application Publication No. 2011/0168446A1 and U.S. Pat. No. 7,145,473 B2 describe examples of electromagnetic telemetry for communicating signals between equipment disposed at the Earth's surface and LWD and/or MWD instruments in a wellbore. One type of electromagnetic telemetry known in the art includes an electric dipole antenna formed by an insulated gap between conductive segments on a drill collar associated with the MWD and/or LWD instruments, or by a toroid disposed on the outer surface of the collar. A time varying voltage impressed across the insulated gap (or the toroid) generates an electromagnetic field which can be used to communicate between a surface electric dipole receiver antenna or a plurality of differently oriented electric dipole antennas and the MWD/LWD instrument. See the basic concept shown in FIG. 1. A surface transceiving sensor, i.e., a dipole antenna, may be composed of an electrode placed in the ground a selected distance from a drilling unit or “rig” and the well casing. The voltage between the electrode and the well casing is measured and signals from the MWD and/or LWD instruments encoded into the transmitted electromagnetic field are decoded from the measured voltages. Conversely, voltage imparted across the casing and electrode may induce an electromagnetic field in the subsurface that is detectable by the dipole antenna or toroid on the MWD/LWD instrument and internally decoded. The foregoing signal communication from wellbore to surface may be referred to as “uplink” communication and the surface to wellbore communication may be referred to as “downlink” communication.
Referring to FIG. 1, an electromagnetic telemetry system known in the art is shown to explain the basic components thereof. U.S. Patent Application Publication No. 2011/0168446A1 and U.S. Pat. No. 7,145,473B2 referred to above describe adaptations of the system shown in FIG. 1 and may be referred to for more details. A drilling unit or “rig” is shown generally at 28 and has equipment (not shown separately) to raise, lower and rotate a drill string 18 with a bottom hole assembly (BHA) 23 at its lower end. A drill bit 26 is disposed at the longitudinal end of the BHA 23 and is rotated, either by the drilling unit 28 and/or a motor (not shown) in the drill string 18 to axially extend the length of a wellbore 11. When the wellbore 11 is initialed drilled to a selected depth, a casing 14 may be inserted into the wellbore 11 and cemented in place. Drilling may then resume. The BHA 23 may include an MWD and/or LWD instrument, shown generally at 23A. The MWD/LWD instrument may be any type known in the art and may include sensors (not show separately) for measuring orientation of the BHA 23, as well as sensors for measuring shock and vibration of the BHA 23, and/or sensors for measuring one or more physical parameters of the formations (including conductive layers 16 and a reservoir formation 20) through which the instrument 23A passes during drilling and any subsequent movement within or along the wellbore 11. Such physical parameters may be of any kind known in the art, and may include, without limitation, electrical resistivity, acoustic velocity, natural gamma radiation, spectrally analyzed natural gamma radiation, density, neutron porosity and/or capture cross section, and nuclear magnetic resonance relaxation times. The foregoing are only examples and in no way are intended to limit the scope of the present disclosure.
In the present example circuitry (not shown) in the instrument 23A may be used to impart a time varying voltage across an insulating gap 24 disposed between conductive components 22, 22A of the instrument 23A. The circuitry (not shown) in the instrument 23A may include devices to encode measurements from the various sensors (not shown) in the instrument into the time varying voltage. The imparted voltage generates a time varying electromagnetic field in the formations 20, 16 which includes the encoded measurement data. In the present example, a voltage induced between an electrode 12 inserted into the ground at a selected distance from the drilling rig 28 and the casing 14 may be measured and decoded into data encoded into the time varying voltage by a surface measurement and decoding system, shown generally at 10 and which may be of any type known in the art.
However, the amplitude of the electromagnetic field induced by the instrument 23A is highly attenuated before it reaches the surface. Research has found that limiting factors for the received signal amplitude include the subsurface formation resistivity distribution, drilling fluid electrical resistivity and the depth of the MWD/LWD instrument in the wellbore. All of the foregoing may cause substantial transmitted signal attenuation such that reliable data communication may be difficult to obtain. Electromagnetic telemetry is preferred over drilling fluid flow modulation (“mud pulse”) telemetry not only because it works in certain conditions where mud pulse telemetry does not, e.g., underbalanced drilling or when the gas or air is used as the drilling fluid, but more importantly, higher data communication rate can be achieved if the signal attenuation can be adequately reduced.
FIGS. 2A and 2B show examples of how conductive formations overlying the instrument (23A in FIG. 1), called “overburden”, attenuate the amplitude of the voltage detectable at the surface. The attenuation is sensitive to the frequency of the time varying voltage imparted across the insulating gap (24 in FIG. 1) and thus, depending on certain conditions may limit the available rate at which data may be communicated between the surface and the instrument (23A in FIG. 1). Conductive drilling fluid in the wellbore and conductive formations are two of the most significant barriers to high data rate signal communication. FIG. 2A shows the case where the formations overlying the wellbore in which the MW/LWD tool have a resistivity of about 2 ohm-m, and FIG. 2B shows the case for overlying formations having resistivity of about 8 ohm-m.